Writing in one of Italy’s major secular newspapers, Pope Francis called for a “sincere and rigorous dialogue” between the church and nonbelievers as an “intimate and indispensable expression” of Christian love.

The Pope’s words appeared in a 2,600-word letter published in the September 11 edition of the Rome daily La Repubblica, in reply to recent articles by Eugenio Scalfari, a co-founder and former editor-in-chief of the newspaper.

An “open and unprejudiced dialogue” between Christians and those of no religious faith is “rightful and precious” today for at least two reasons, Pope Francis wrote.

Such a dialogue could “open doors for a serious and fertile encounter” between secular culture and Christian culture, which have lost the ability to communicate due largely to modern views of faith as the “darkness of superstition opposed to the light of reason”.

In fact, the Pope wrote, the impulse toward communication springs out of the very nature of Christian faith.

“Since it is born of love,” he wrote, quoting his own encyclical Lumen Fidei, “faith is not intransigent, but grows in respectful coexistence with others… Far from making us inflexible, the security of faith sets us on a journey; it enables witness and dialogue with all.”

This loving quality of faith offers a path of dialogue with sceptics, despite modern ideas of truth as “relative and subjective,” he added.

“I would not speak, not even for a believer, of ‘absolute’ truth, in the sense of absolute as disconnected, lacking any relationship,” the Pope explained.

“Truth, according to Christian faith, is the love of God for us in Jesus Christ. Therefore truth is a relationship.

“This does not mean that truth is variable and subjective – on the contrary,” he wrote. “But it means that truth is given to us always and only as a path and a life… In other words, truth being after all one in the same as love, it requires humility and openness in order to be sought, welcomed and expressed.”

Asked whether the Church condemns those who lack and do not seek religious faith, the Pope replied that the “mercy of God is unlimited if directed to someone with a sincere and contrite heart”.

“The question for someone who does not believe in God lies in obeying one’s own conscience,” he wrote. “Sin exists, even for one who does not have faith, when one goes against conscience. To listen to and obey it means, in fact, to choose between what one perceives as good or as bad. And on this choice is staked the good or evil of our action.”

In his letter, Pope Francis also distinguished the proper roles of the “religious sphere and the political sphere” of society.

The Church is “called to sow the leaven and salt of the Gospel, that is the love and mercy of God that reach all men,” he wrote, whereas to “civil and political society falls the arduous task of articulating and incarnating – in justice and solidarity, in law and peace – an ever more humane way of life”.

It is extremely rare for a pope to contribute to a secular newspaper. In December 2012, the Financial Times published an article on the meaning of Christmas by Pope Benedict XVI.